1) The effect of L-carnitine on the rate of oxidation of 1-14C-palmitic acid by human skin fibroblasts and skeletal muscle cells grown in culture was studied. Incubation of cells for 24 hrs with 0.5-10 mM L-carnitine increased the rate of substrate oxidation to CO2 by up to 50 percent in normal fibroblasts and by as much as 100 percent in muscle cells. Fibroblasts from two patients with systemic carnitine deficiency and two patients with adrenomyeloneuropathy responded to L-carnitine in a manner similar to control fibroblasts. Uptake of 14C-carnitine by skin fibroblasts from the two patients with carnitine deficiency was not lower than the uptake of control cells. 2) A study of metabolism of very long chain fatty acids in human cells in culture was initiated. 3) It was shown that dexamethasone consistently stimulates oxidation of labeled palmitic acid in fibroblasts. 4) Hybrid cells were produced by fusion of mouse myeloma and spleen lymphocytes from a mouse immunized against human skin fibroblasts with the aim of isolating cells secreting antibodies against LDL receptors.